Ascorbic acid and its various derivatives are known as compounds exhibiting efficacy and effects in whitening action, anti-oxidant action, collagen synthesis promotion action, etc., and they are contained in medicines, cosmetics, feeds and the like.
Of the ascorbic acid derivatives, compounds wherein a hydroxyl group at the 2-position is esterified with phosphoric acid and a hydroxyl group at the 6-position is esterified with a higher fatty acid and their salts are hardly oxidized and are stable and amphipathic. Therefore, these compounds have good affinity for living organisms and rapidly penetrate into the organism tissues such as the skin, so that application to medicines, cosmetics, feeds, etc. is expected.
However, when agents for skin external use are prepared using the salts of higher fatty acid esters of ascorbic acid-2-phosphoric acid ester, decomposition of these compounds takes place in the resulting agents, and besides, turbidity or precipitation occurs with time, resulting in a problem of markedly impaired appearance.
In connection with the above, there has been disclosed a whitening agent for skin external use, which is improved in the stability by allowing cyclodextrin to include ascorbic acid or a higher fatty acid ester of ascorbic acid and which uses a polyhydric alcohol as a solvent (Japanese Patent Laid-open Publication No. 113525/1996). In Japanese Patent Laid-open Publication No. 113525/1996, however, there is no description about an agent for skin external use which uses a higher fatty acid ester of ascorbic acid-2-phosphoric acid ester or its salt.
In Japanese Patent Laid-open Publication No. 348228/2002, there is disclosed a composition for skin external use in which ascorbic acid, its ester derivatives, its ether derivatives or their salts are kept in a stable state in aqueous media composed of water and glycol ethers. In this document, however, there is no description about an agent for skin external use which uses a salt of higher fatty acid ester of ascorbic acid-2-phosphoric acid ester.
In Japanese Patent Laid-open Publication No. 3330/2002, there is disclosed a cosmetic which comprises a water-soluble ascorbic acid derivative, such as ascorbic acid phosphoric acid ester magnesium salt, water and 1,2-alkanediol and is stable in spite of passage of time. In this document, however, there is no description about an agent for skin external use which uses a salt of higher fatty acid ester of ascorbic acid-2-phosphoric acid ester.
Further, the present inventors have already proposed an agent for skin external use and a cosmetic in Japanese Patent Laid-open Publication No. 176217/2003. This agent for skin external use contains a salt of higher fatty acid ester of ascorbic acid-2-phosphoric acid ester and has been adjusted to pH 7 to 9 to inhibit decomposition of the salt in the agent and thereby improve stability and solubility.
In the working example in this patent document, a lotion obtained by adding 10% by mass of propylene glycol to an agent for skin external use containing a salt of higher fatty acid ester of ascorbic acid-2-phosphoric acid ester is disclosed, and it is described that decomposition of the salt of higher fatty acid ester of ascorbic acid-2-phosphoric acid ester is inhibited. In this lotion, however, occurrence of turbidity could not be inhibited though decomposition of the salt of higher fatty acid ester of ascorbic acid-2-phosphoric acid ester could be inhibited. Accordingly, from the viewpoint of practical use of an agent for skin external use containing a salt of higher fatty acid ester of ascorbic acid-2-phosphoric acid ester, development of an agent for skin external use which is more effectively prevented from occurrence of turbidity or precipitation with time is still eagerly desired.